1. Field of the Invention
The present invention relates a control device for an automatic transmission for a vehicle and particularly to a method, in an automatic transmission for a vehicle provided with a torque converter between an output shaft of an engine and an input shaft of a gear type transmission and provided with a lockup clutch for mechanically and directly engaging an input shaft with an output shaft of the above torque converter, for controlling a lockup force of the above lockup clutch and more particularly to an improvement in lockup force control during acceleration from a deceleration driving zone.
2. Description of the Related Art
It is well known that transmission efficiency of a torque converter is improved by providing a hydraulic lockup clutch that can mechanically and directly engage an input shaft with an output shaft of a torque converter therein and by locking the above lockup clutch in a conventional automatic transmission for a vehicle.
Also, by locking this lockup clutch during deceleration so as to restrain rapid lowering of engine rotation by transmitting a reverse driving force from the driving wheel side to the engine side, a fuel supply stop control is carried out for a long time during deceleration driving so as to improve fuel consumption (See Japanese Unexamined Utility Model Publication No. 62-115556).
In the meantime, when lockup control is executed during deceleration driving and particularly when an accel pedal is depressed the low car-speed side in a deceleration driving zone where vehicle speed or engine rotation speed is more than a predetermined value and the engine is in the non-load state, rapid fluctuation in engine output torque is transmitted to the wheels through the lockup clutch, which imports unpleasant vibration to passengers in the vehicle. Thus, there is the problem that the set vehicle speed at which the lockup control is to be executed cannot be significantly lowered.
The applicant then considered lowering the lockup force of a lockup clutch in a deceleration driving zone more than that in a non-deceleration driving zone so as to buffer a rapid rise in the engine output torque during acceleration from the deceleration driving state by the above lockup force, which was lowered in advance.
In order to favorably buffer the above rapid increase in torque with the torque converter, it is only necessary to substantially reduce the lockup force of the lockup clutch in the deceleration driving state. However, when the lockup force during deceleration driving is lowered excessively, the object of increasing the transmission efficiency during deceleration driving could not be fully attained, and even if the lockup force during deceleration driving is fully lowered, the lockup force was immediately increased to a level corresponding to non-deceleration driving and a rise in torque could not be favorably absorbed, which may impart an unpleasant vibration to passengers.